Conventional apparatus used to treat substrates with ions include beamline ion implanters and plasma immersion ion implantation tools. Both are appropriate for implanting ions over a range of energies. In beamline ion implanters ions are extracted from a source, mass analyzed and then transported to the substrate surface. In plasma immersion ion implantation apparatus, a substrate is located in the same chamber the plasma is generated adjacent to the plasma. The substrate is set at negative potential with respect to the plasma and ions that cross the plasma sheath in front of the substrate impinge on the substrate at perpendicular incidence angle. Recently a new processing apparatus that allows control of extracted ion angular distribution (IAD) has been developed. In this apparatus, ions are extracted from a plasma chamber but unlike the beamline where the substrate is located remotely from the ion source, the substrate is located proximate the plasma chamber. Ions are extracted through an aperture of special geometry located in an extraction plate that is placed proximate a plasma. Changing the geometry of the aperture allows changing of the ion angular distribution, i.e., the mean angle and angular spread of the ion distribution. This may be appropriate to treat substrates that present surface features whose sidewalls are to be exposed to ions, for the purposes of implantation, etching, or other processing. In order to treat such sidewalls, ions are extracted through the aperture of a shape and size to generate a desired ion angular distribution. However, it may be desirable to provide further control in a plasma system over ion angular distribution in order to process substrates more effectively. It is with respect to these and other considerations that the present improvements have been needed.